The present invention relates to a control for electrical positioning drives with variable speed, and more particularly to a control for sewing machine positioning drives, with a range of adjustment providing the given nominal value of the speed and a range of adjustment for at least one additional signal in digital form.
In the case of a known, optoelectronic control of this general type, not only the additional signals, but also the signals providing the given nominal value of the speed are produced in digital form. The non-contact production of the signals is actually of advantage in this case. However, with an increasing number of the discrete nominal values of the speed, i.e., a decrease of the increments between the individual nominal values, the cost of this control appreciably increases owing to the necessary enlargement of the optoelectronic elements.
It is also known for the non-contact production of the nominal values of the speed, for example, by West German Pat. No. 2,135,777, to utilize a sound generator and a magnet adjustable vis-a-vis the generator. In this instance, the nominal value of the speed is infinitely variable. The nominal value is, however, dependent on the mechanical tolerances and on the temperature. Furthermore, switches must be provided for the production of the additional digital signals which clearly increase the cost, particularly because of the required adjustment. This is also applicable to another known control disclosed in West German Pat. No. 1,763,853, in the case of which the non-contact and continuous production of the nominal values of the speed is effected by means of a transmitter with an adjustable coupling factor.
In the case of another control described in West German Pat. No. 1,640,113, the nominal values of the speed as well as the additional signals are produced by means of several flat spring contact units. The disadvantage of this control is the considerable expenditure for the adjustment of the flat spring contact units as well as the dimension and number of the flat spring contact units which increase with the continuous decrease of the increments between two successive nominal values of the speed.